Flame ionization detectors operate by burning the material to be analyzed so as to form ions and measuring the current that is produced when the ions are collected. When used with a gas chromatograph, the gases eluting from the column are mixed with hydrogen or some other highly flammable gas and passed through a tube to form a jet. Air is introduced at the periphery of the jet so as to form a stable flame when the jet is ignited. A collector tube is placed adjacent to the jet, a source of direct current voltage and an electrometer are connected in circuit with the tubes so as to collect the ions formed in the flame and produce a current in the electrometer that is proportional to the rate at which the ions are collected.
When used with a chromatograph in which the sequence of operations is controlled by an attendant, the jet can be ignited with manual means whenever required and it can be reignited if the jet flames out, but when the detector is used in a chromatograph that operates in response to a software program, it is essential that initial ignition of the jet, as well as its re-ignition, be achieved by automatic means.